Three columns, three answers
NM-B cable is held to the 60 °C column, THHN in conduit on 60/75 °C breakers uses 75 °C. That is why 12 AWG NM-B and 12 AWG THHN can both serve 20 A.
What size wire for 20 amps?
A 20 A circuit takes 12 AWG NM-B in the wall or 12 AWG copper THHN in conduit (75 °C), with a 12 AWG ground; aluminum needs 10 AWG. At an 16 A load the copper reaches 113 ft at 240 V (3 % drop). Used for kitchen, bathroom, laundry and garage receptacles.
02 · Ampacity · NEC 2023 Table 310.16
12 AWG copper · 90 °C insulation, 75 °C terminals, 30 °C, 3 conductors
USE:
12 AWG COPPER · 25 A ALLOWABLE · 20 A MAX BREAKER
Allowable
25 A
12 AWG Cu
Max breaker
20 A
240.4(D) cap
For your load
12 AWG
20 A
12 AWG copper carries 30 A after derating from the 90 °C column and 25 A at its 75 °C terminals, so it is good for 25 A on a breaker up to 20 A.
- Meets NEC 2023 Table 310.16 at 75°C — verify with your local AHJ/inspector110.14(C)
- Load 20 A ≤ 25 A310.14
Derating math
- Table 310.16 at 90 °C
- 30 Aderating starts at the insulation rating
- Ambient factor
- × 1.0030 °C, Table 310.15(B)(1)
- Bundling factor
- × 1.003 current-carrying conductors, 310.15(C)(1)
- Derated
- 30 A
- Terminal limit (75 °C)
- 25 A110.14(C)
- Allowable
- 25 Athe terminals decide
12 AWG
- Copper 60 / 75 / 90 °C
- 20 / 25 / 30 ANM-B uses the 60 °C column (334.80)
- Aluminum 60 / 75 / 90 °C
- 15 / 20 / 25 A
- Largest breaker (Cu, 75 °C)
- 20 A240.4(D) small-conductor cap
- Solid diameter
- 0.0808"2.053 mm · ASTM B258 formula
- Stranded (7 strands)
- 0.0920"2.337 mm · Table 8
- THHN outside
- 0.130"3.30 mm · 0.0133 in² · Table 5
- Area
- 6,530 cmil3.31 mm² · nearest IEC 4 mm²
- DC resistance, 75 °C
- 1.98 Ω/1000 ftaluminum 3.25 Ω/1000 ft · Table 8
- Copper weight
- 19.8 lb/1000 ft
20 A circuit at a glance
Wire
- Copper NM-B (60 °C)
- 12 AWG
- Copper THHN (75 °C)
- 12 AWG
- Aluminum (75 °C)
- 10 AWG
Ground & breaker
- Copper ground
- 12 AWG
- Aluminum ground
- 10 AWG
- Continuous load max
- 16 A
- Breaker
- 20 A
Distance (3 %)
- Cu at 120 V, 16 A
- 56 ft
- Cu at 240 V, 16 A
- 113 ft
- Al at 240 V, 16 A
- 110 ft
20 A circuit tables 7 rows
Sort any column, print a shop sheet, download CSV. Click a row to load it in the calculator.
| Actions | |||||
|---|---|---|---|---|---|
| 12 AWG | 12 AWG | 12 AWG | 113 ft | ||
| 12 AWG | 12 AWG | 12 AWG | 113 ft | ||
| 10 AWG | 10 AWG | 10 AWG | 110 ft |
Feet one way (the current goes out and back). Ampacity is a separate check: a long run may need a heavier wire for drop, never a lighter one for heat.
05 / Field guide
20 amps:
which wire, which method.
A 20 A circuit takes 12 AWG NM-B in the wall or 12 AWG copper THHN in conduit (75 °C), with a 12 AWG ground; aluminum needs 10 AWG. At an 16 A load the copper reaches 113 ft at 240 V (3 % drop). Used for kitchen, bathroom, laundry and garage receptacles.
Breaker rules
240.4(D) fixes the small sizes: 14 AWG on 15 A, 12 AWG on 20 A, 10 AWG on 30 A, whatever the ampacity table allows.
Distance
The answer above is the minimum for heat. On a long run upsize for voltage drop, and grow the ground with it: the table below gives the longest 240 V run at 3 % for each candidate size.
Questions
Answered plainly.
01What size wire for a 20 amp breaker?
+−
12 AWG copper NM-B in the wall, 12 AWG copper THHN in conduit or 10 AWG aluminum, all with ampacity at least 20 A (NEC 2023 Table 310.16).
02Can I use 12 AWG wire on a 20 amp breaker in NM-B cable?
+−
Yes: 12 AWG NM-B is rated 20 A at 60 °C.
03What size ground wire for 20 amps?
+−
12 AWG copper or 10 AWG aluminum (Table 250.122), larger in proportion if the circuit wire is upsized for distance (250.122(B)).
04How far can I run a 20 amp circuit?
+−
At 16 A (80 %) on 12 AWG copper, 113 ft one way at 240 V or 56 ft at 120 V before the drop passes 3 %. Beyond that, go up a size.
05What is the largest continuous load on a 20 amp circuit?
+−
16 A: a continuous load (3 hours or more) may use only 80 % of the breaker rating (the 125 % rule).